1. Field of the Invention
The present invention relates to methods and apparatus for probing of an IC (integrated circuit) device with a dual-laser voltage probe.
2. The Prior Art
Optical-probe test systems typically operate by focusing a laser beam using lenses onto an IC device under test (DUT) and detecting the reflected beam. The laser-beam sampling pulses are generated at a wavelength at which silicon is effectively transparent. A confocal optical system allows a shallow focal plane to be precisely positioned relative to structure of the DUT to be investigated.
FIG. 1 illustrates schematically the principal of a prior-art confocal laser-beam probe system. Laser source 100 generates an incident beam 105 which passes through a beam splitter 110 via steerable galvo-mirrors 115, a scan lens 120 and an objective lens 125 to a DUT 130. The beam confocally-reflected from a focal plane 135 is shown in solid lines 140, while the non-confocally-reflected beam is shown in dashed lines 145. These reflected beams pass via objective lens 125, scan lens 120, galvo-mirrors 115, beam splitter 110 and lens 150. Confocal aperture 155 allows the confocally-reflected beam to pass to detector 160, but intercepts the non-confocally-reflected beam. Detector 160 thus sees only the beam confocally reflected from focal plane 135 of DUT 130.
FIG. 2 shows schematically the manner in which the probe in a confocal laser-beam probe system is scanned in three dimensions to examine regions of interest within a sample. Galvo-mirrors 115 are steerable about two axes in the .phi. and .theta. directions so as to scan the incident beam in X- and Y-directions in a raster scan pattern 200 over a scan-lens back focal plane 205. The incident beam is thus scanned in a corresponding demagnified pattern 210 over the DUT 130 after passing through objective lens 125. A beam expander 210 comprising a pair of lenses 215 and 220 and a pinhole aperture 225 provide spatial filtering of the incident laser beam 105 to approximate point source illumination. The incident beam is focused in the Z-direction by moving the sample as indicated at 230.
Due to DUT interactions with the beam, the reflected beam carries information about the DUT. This information can take the form of reflected beam amplitude, phase and/or polarization modulations, for example. These modulations may be due to changes in the DUT charge state, electric field strength, and/or temperature, caused by application of a test pattern to the DUT, for example. In order to detect phase and polarization modulations, it is usual to first convert them into an amplitude modulation. This can be done for polarization modulations by passing the beam through a beam polarizer. For a phase modulated beam, this can be done using interference techniques. Amplitude modulations can then be detected as intensity modulations using a photodetector. To increase the measurement bandwidth, equivalent time sampling techniques are used in which either the laser or the photodetector is pulsed/gated and the DUT test pattern is made repetitive.
Problems arise when effects other than the one being probed result in unwanted modulations of the beam amplitude. For example, temperature changes as well as electrical field strength changes in the DUT may cause amplitude changes in the reflected probe beam. If only electrical field strength changes are of interest, the temperature changes behave as a source of noise in the measurement.
Other sources of noise may be present. FIGS. 3A-3C illustrate one source caused by fluctuations in the overlap between the incident probe beam spot and the active regions of the DUT under study. This might result from movement of the beam focusing lenses relative to the DUT, caused by external mechanical vibrations, for example. It might also result from electronic noise in the drivers to the beam steering optics. FIG. 3A illustrates this variation in overlap 300 between the laser probe beam spot 305 and a region 310 to be probed, as might be seen when looking along the beam direction at the DUT. A sinusoidal variation 315 of overlap vs. time is shown for simplicity; the actual motion and resulting noise variation are likely to be more complicated. FIG. 3B illustrates at 320 the effect of noise due to overlap variation on the reflected intensity of the beam, assuming that the active region of the DUT has a higher reflectivity than its surroundings. FIG. 3C illustrates the reflected intensity variation 325 due to both the noise 320 and the signal of interest (e.g., charge density variation in the DUT due an applied test pattern).
If the noise is random with respect to the test pattern, it is common practice to time-average the measurements over a number of test pattern cycles to increase the signal-to-noise ratio (SIN). However, it may take a large number of samples to average out the noise sufficiently since the signal-to-noise ratio is proportional to the square-root of the number of measurements averaged, i.e., S/N .alpha. sqrt(n). For example, if the noise fluctuations are initially 100 times larger in amplitude than the signal (S/N=0.01), it would take 10,000 samples to increase the signal to noise level to 1 (0.01.times.sqrt(10,000)=1). Since the interactions of interest may only modulate the reflected beam intensities by 0.01, a noise amplitude of just 10% (S/N=0.001) is problematic.
Improved schemes are need to compensate for much of this noise, such that the number samples to be averaged to obtain adequate signal-to-noise ratio is reduced.